The following is an excerpt from Death & Sex[1] by Tyler Volk[2] and Dorion Sagan[3]. It has been adapted for the Web.

From chapter 9: Tuning Longevity

The point is not that creatures with negligible senescence live so much longer than humans—they might or might not. The issue is that in these species the individuals who are longest-lived (so far as we know) do not degrade from internal causes that would make their chances of death increase as they age. Some species with negligible senescence maintain high reproductive output despite their increasing years. In fact, among lobsters, another candidate species with negligible senescence, egg laying can become more copious with age.

Birds are turning out to be flashy mentors to the biogerontologists who patiently seek answers to deep questions about the spectrum of longevity in animals. Here’s a perplexing finding that violates the general rule that large creatures tend to live longer than small creatures: A typical mouse of twenty grams lives about three years, while a canary of the same weight lives for twenty years, almost seven times as long. Limited data from wild birds show they live almost twice as long as same-weight mammals in captivity, while captive birds live about three times as long as captive mammals of the same weight. Some of the numbers are extraordinary. Scarlet macaws, for example, have been known to live more than ninety years, which is about four times the life span of average, similar-size birds and twelve times the mammal average at the same weight.

The explanation for avian longevity comes from what is known as the evolutionary theory of life span. Biogerontologist Steven Austad and his colleague Donna Holmes use the phrase Fly now, die later to describe it. The motto applies not just to birds but to bats, too. On average across a range of body weights, bats live about three times longer than other mammals of comparable weight. Austad and another colleague, Kathleen Fischer, hypothesize that the aerial abilities of birds and bats make them much less vulnerable to predators than are ground mammals. Austad and Fischer further reason that any mammal that can sail between trees should be better than ground dwellers at avoiding predation. They surveyed data for gliding species of mammals: three squirrels, five marsupials, and one flying “lemur.” Taken together, these species have life spans that average 1.7 times the mammalian average for their weights. In another study, all marsupial mammals were lumped into two groups: tree-dwelling or ground-dwelling (species using both habitats were ignored). For comparable weights, the average life spans of arboreal species beat terrestrial ones by nearly 60 percent, Austad and Fischer found.

The core concept in the evolutionary theory of life spans is that creatures that are less vulnerable to predators are more likely to have evolved a healthy dose of maintenance and repair abilities inside their bodies. This is a central tenet of the overall evolutionary logic that relates life span to lifestyle: The intrinsic capacity of an animal’s bodily metabolism to produce longevity is evolutionarily tuned to the odds that the animal will or will not be able to live long, on average, based on the relative kinds of advantages or disadvantages that its lifestyle confers on its survival.

From chapter 11: Death-Denying Defenses

What happens when people are reminded that they eventually are going to die, that they are mortal? Epicurus and Buddha encouraged their followers to remind themselves, as constant meditation. Thinking about mortality, they and many others have claimed, is a path to deep contentment, even happiness.

But do we need to remind ourselves? The world we live in seems to do a great job of that already. We are exposed to the fact of mortality all the time—movies (what’s a movie without death or near-death?), news (“if it bleeds, it leads”), the health problems of family and friends. Sometimes living within a death-soaked world can be disturbing. But after all, it is the state of nature with food webs and biogeochemical cycles. Almost inevitably, this state has been stepped up a notch by the ability of humans to symbolize, fantasize, and hypothesize, not to mention our recognition of real murder and genocide. What more can be known? In recent years some experiments in the field of social psychology called “terror management theory” have been yielding some fascinating insights on how the basic knowledge of our mortality affects us.

If you happen to be a student in an introductory psychology course at a college or university, you are usually required to participate in one or more experiments. Undergraduate psychology students are needed to form a large enough pool of “lab rats” to provide statistically relevant results. If your school was one that had experiments in terror management theory (among a large number of other types of experiments), you might find yourself in a room at a desk filling out a “personality questionnaire.” At least, that is what you are told.

You are told nothing of the true purpose of the overall experiment. Instead, you might be informed that the questionnaire is about personality traits and interpersonal judgments. Scattered among a range of questions to answer are the following: “Please briefly describe the emotions that the thought of your own death arouses in you,” and “Jot down, as specifically as you can, what you think will happen to you when you die and once you are physically dead.” …